The present invention relates to cleaning openings in well liners positioned adjacent fluid producing formations using high velocity liquid jets and, more particularly, the present invention relates to methods and apparatus for use in removing plugging material from openings in oil well liners and the like with liquid jets having velocities in excess of 700 feet per second which are directed at the well liners through orifices having a standoff distance between the end of the orifice and the well liner of between five and 10 times the diameter of the orifice.
In the well producing art it is customary to complete an oil well or a water well adjacent a fluid producing formation by inserting a metallic well liner. Openings in the well liner provide passageways for flow of fluid such as oil or water and other formation fluids and material from the formation into the well for removal to the surface. However, the openings which, for example, may be slots preformed on the surface or perforations opened in the well, will often become plugged. This problem is especially serious in areas of viscous oil production from unconsolidated sand formations. Since it is highly desirable to prevent sand from entering the well, the liners used in this type of formation are often completed with narrow longitudinal slots. The slots prevent the entry of most of the sand with the oil, but in time they become plugged. Of course plugging is not limited only to slotted liners but also occurs in perforated liners even though the perforations may be considerably larger in size than the slots. In any event, removal and replacement of the liner is costly and is only a temporary solution since the liner will eventually again become plugged.
Sections of recovered plugged liners have been analyzed to determine the identity of the plugging material. Results have shown that the plugging material is mostly inorganic. Generally, it appears to be fine sand grains cemented together with oxides, sulfides and carbonates. Some asphaltenes and waxes are also present. Where water is produced, scale also seems to be present and presents a very tough plugging material.
Many methods for cleaning openings in well liners have been heretofore suggested. These methods include pumping a fluid between two or more opposed washer cups until the pressure builds up sufficiently to hydraulically dislodge the plugging material. Explosives such as primer cord (string shooting) have been used to form a high energy pressure shock wave to hydraulically or pneumatically blow the plugging material from the perforations. The disadvantages of these two methods are that the energy is applied nondirectionally to the liner and it always takes the path of least resistance. The use of these methods generally results in opening only one or two perforations out of a perforation row containing from 16 to 32 perforations.
Other prior art methods of cleaning the openings in liners include the use of mechanical scratches and brushes to cut, scrape or gouge the plugging material from the perforations. There are many disadvantages of these approaches. For example, the knives or wires in the brushes must be very thin to enter the slotted perforations which generally measures only 0.040 to 0.100 inches wide and, therefore, the knives and wires are structurally weak. Thus an insufficient amount of energy is generally applied to really unclog the perforations. Furthermore, the cleaning tool must be indexed so that the knives or wires actually hit a perforation. Since only 3% of the liner surface area is generally perforated, the chances are not favorable for contacting a perforation.
The use of chemicals such as solvents and acids have been used to dissolve the plugging materials. These are major disadvantages to the chemical approach. Thus the material plugging the perforations varies widely even in a well which requires a number of different chemicals to solubilize them. The combinations of plugging materials often inhibits the reaction of the chemicals. For example, an oil film will prevent an acid from dissolving a scale deposit and a scale deposit will prevent a solvent from being effective in dissolving heavy hydrocarbons. The chemicals cannot always be selectively placed where they are needed due to varying permeabilities encountered in a well bore and/or they dissolve the material in a few perforations and then the chemicals are lost into the formation where they can no longer be effective in cleaning the perforations.
Jetted streams of liquid have also been heretofore used to clean openings. The use of jets was first introduced in 1938 to directionally deliver acid to dissolve carbonate deposits. Relatively low velocities were used to deliver the jets. However, this delivery method did improve the results of acidizing. In about 1958 the development of tungsten carbide jets permitted including abrasive material in a liquid which improved the ability of a fluid jet to do useful work. The major use of abrasive jetting has been to cut notches in formations and to cut and perforate casing to assist in the initiation of hydraulically fracturing a formation. The abrasive jetting method requires a large diameter jet orifice. This large opening required an unreasonably large hydraulic power source in order to do effective work. The use of abrasives in the jet stream permitted effective work to be done with available hydraulic pumping equipment normally used for cementing oil wells. However, the inclusion of abrasive material in a jet stream was found to be an ineffective perforation cleaning method in that it enlarged the perforation which destroyed the perforation's sand screening capability.
There is, therefore, still a need for a method of cleaning openings in a well liner which results in cleaning substantially the entire opening and which is a practical and relatively easy operation to perform. Further, there is need for a method of cleaning openings in such liners which does not destroy or alter the openings or damage the liner.